Enhanced Resolution in EPR Spectroscopy Using <i>para</i> ‐Hydrogen Matrices

We developed a matrix isolation experiment utilizing a closed-cycle helium cryostat, which operates at 2.5 K and enables electron paramagnetic resonance (EPR) measurements in solid para-hydrogen (p-H₂) matrices. The EPR spectra of the persistent 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) radical and an in situ generated P-centered mono-radical were recorded at 2.5 K in p-H₂. The spectrum of TEMPO in p-H₂ shows a narrower linewidth compared to argon, and its matrix-isolated spectrum and simulation are reported. The P-centered mono-radical was generated in p-H₂ via in situ photolysis of the corresponding phosphorus iodide, which affords roughly a three-fold increase in spectral resolution compared to argon, as well as higher sensitivity due to the non-existing cage effect in soft p-H₂.